The possibility of reconstructing images of objects (in particular, human beings) hidden by smoke and flames in fire scenes is of particular importance, especially in military and homeland security situations, in industrial sites and generally in the security field.
White-light detectors and visible wavelength sensors can not be used to this purpose, because wavelengths in the visible spectrum undergo strong scattering by smoke particles.
On the other hand, thermographic detectors employing, for example, an array of bolometers operating in an infrared wavelength region (e.g. 7-14 μm) are able to acquire images of objects hidden by smoke. Such detectors allow clear vision (in particular with laser IR illumination) through smoke, since radiation in the infrared region is scattered just slightly by smoke particles.
However, thermographic detectors are not capable of providing images of objects hidden by flames. Such detectors indeed typically comprise an objective lens which focuses radiation emitted or scattered by the object onto the thermographic detector. Such objective lens disadvantageously focuses, on at least some of the bolometers, also the infrared radiation emitted by flames and matching the numerical aperture of the lens. Such focused radiation disadvantageously may induce saturation of the bolometers and in any case hides the contribution provided by the object, so that the reconstructed image exhibits blind areas where the object is hidden by flames.
Thermal imaging cameras are also known, which are typically used for monitoring furnaces or boiler equipment used in industrial sites, so as to promptly detect possible problems and prevent their failures. These known cameras use a spectral waveband filter that only allows the detection of thermal radiation within specific wavelengths where no hot gases are emitted, e.g. around 3.80 μm.
Such thermal imaging cameras equipped with such filters can not be used to see through all kinds of flames, e.g. the flames generated during the combustion of a forest or building. In fact, solid particles of incandescent soot typically emit radiation in a continuous spectrum over a wide region extending from visible to infrared, and also at around 3.8 μm. Also such cameras then suffer from the above described saturation effect.